Perforated metal plates are useful as filter media for mechanical filters. Filters of this type operate on the basis that the filter medium works as a simple porous barrier screen, removing and retaining solid particles too large to pass through the medium, but allowing the fluid in which the particles are suspended and particles smaller than the pores to pass through. These filters in effect provide direct interception of solids, with only a minor involvement of phenomena such as depth filtration which are major contributing mechanisms in other types of filtration. The intercepted particles are stopped at the upstream surface of the perforated plate, their size preventing them from passing through the pores. Surface retention of the particles provides an advantage in that the particles may be readily removed, allowing for re-use of the plate medium. Perforated metal plates also offer potential advantages in their strength and resistance to corrosion and capability for service in high-temperature fluids or in high-pressure applications as compared with other types of filter media.
While potentially favorable aspects of perforated plate filter media are well-known, practical limitations have been imposed on their use owing to the difficulty of fabricating them with desired perforation sizes. As stated in Solid/Liquid Separation Technology by Derek B. Purchas, published by Uplands Press Ltd. (1981), at pages 116-117:
The use of perforated metal sheets as filter media is generally of very restricted interest since standard metal working techniques are unable to produce holes of sufficient fineness and regularity, excepting for applications such as in roughing strainers; for example, the finest grade in the very extensive range of Greenings has holes of 0.26 in. (i.e., 660 microns), the porosity of this being 22.6%. PA1 Expanded metal sheets have diamond-shaped apertures even the smallest of which in the so-called Micromesh range typically is 0.75 mm.times.0.60 mm (along its long and short axes, respectively). PA1 From the filtration point of view, the most useful range of perforated metal sheets are those made by electroforming techniques combined with photoetching. This is the basis of the two ranges of nickel products available from Veco, the one being sheets with a maximum size of 200.times.200 mm, with holes as small as 5 microns (tolerance being .+-.2 microns on holes up to 500 microns); such small holes result in very low porosities, down to less than 2%. PA1 The second range comprises sheets of 1 m.times.1 m; the finest grade has 0.04 mm (40 microns) holes, with a porosity of 3.5%, other hole sizes and porosities being 60 microns (8%), 80 microns (14.5%), 100 microns (22.5%), all of these being at a pitch of 0.20 mm with a key thickness of between 0.04 and 0.07 mm. In practice, there are numerous variations of factors such as pitch and shape of hole, result in a wide variety of grades with holes up to 5 mm.
These limitations of perforated plates to large pore sizes and low porosities have served to render perforated plates unsuitable for many important filtration applications. In terms of pore size, pollutants such as various types of dust, liquid mist particles, bacteria, pollens, spores, and other biological materials are smaller in most cases than the available pore sizes so that such materials would pass through such a medium unaffected. In particular, pore sizes down to less than one micron in diameter are needed.
In addition to smaller pore sizes, a higher total porosity is needed for applications requiring a high throughput or involving exposure to pressurized fluids. As stated above, prior perforated metal sheets with pore sizes as small as five microns provide a porosity of less than two percent. A much higher porosity is required for practical applications. Uniformity of pore size and of distribution have also been lacking in prior plates at the smaller sizes. Filter media are rated on their ability to remove particles of a specific size from a fluid. Ideally, the medium would have a precise cut-off point, which refers to the largest particle, normally expressed in microns, which would pass through the filter. In practice, filter media at the smaller sizes have pore sizes extending over a considerable range of values, rather than a single precise value. The ability to selectively remove particles of specific size would be enhanced by providing plates with a controlled, uniform pore diameter. Maximum uniformity of pore shape as well as size is needed to provide for separation of materials within precise, narrowly defined size ranges.
Another important feature needed for perforated plate filter media is a high degree of strength and durability such as to allow the plate to be subjected to removal of particles and to be reused, after undergoing sterilization, chemical cleaning, or the like.
Requirements also exist for perforated plates with properties similar to those discussed above for applications involving an injection or extrusion step. Very small and uniform size pores, consistent with strength and durability, are important for such purposes.